The block diagram of FIG. 1 depicts selected portions of a prior art drive referred to by the general reference character 10. FIG. 1 particularly illustrates those portions of the drive 10 which adapt it for recording digital data on a Compact Disc (“CD”) or DVD.
The drive 10, which is usually incorporated into a digital computer, exchanges digital data with other portions of the digital computer via a computer bus 12. For purposes of the present disclosure, the drive 10 may be understood conceptually as including a control processor 14, although drives 10 may be actually constructed in various other different ways. Responsive to commands which the drive 10 receives via the computer bus 12, the control processor 14, among other things, supervises:                1. rotation of a CD or DVD 16 received into the drive 10 indicated in FIG. 1 by an arrow 18; and        2. operation of an optical subsystem 22 indicated in FIG. 1 by an arrow 26.The optical subsystem 22 focuses light, generated by a laser diode 34, to a spot 36 that is located along a track 38 which spirals inward across the surface of the CD or DVD 16. The control processor 14 operates in fundamentally the same manner for supervising rotation of the CD or DVD 16 and operation of the optical subsystem 22 both while the drive 10 records digital data onto the CD or DVD 16, and while the drive 10 reads previously recorded data from the CD or DVD 16.        
When recording data onto the CD or DVD 16, the control processor 14 may be understood as supplying to an integrated circuit (“IC”) writing current circuit 42:                1. write control data via a writing control bus 44; and        2. serial data for recording along the spiral track 38 via lines that are included in a recorded data bus 46.In turn, the writing current circuit 42 supplies a controlled electrical current to the laser diode 34 via a current output line 48 to generate a temporally changing light beam which the optical subsystem 22 focuses at the spot 36 on the track 38. Heating of the CD or DVD 16 due to the beam of light impinging at the spot 36 alters the physical properties of the CD or DVD 16 thereby recording along the track 38 the digital data which the writing current circuit 42 receives via the recorded data bus 46.        
While recording onto the CD or DVD 16, the energy of the light beam generated by the laser diode 34 must be controlled to heat the CD or DVD 16 at the spot 36 to a precise temperature needed to change the physical properties of the CD or DVD 16. Consequently, the electrical current which the writing current circuit 42 supplies to the laser diode 34 must be precisely controlled responsive to various different recording conditions which include:                1. the physical characteristics of various different types of CDs or DVDs 16 that may be loaded into the drive 10;        2. the speed at which the CD or DVD 16 rotates; and        3. the location of the spot 36 along the spiral track 38.        
The waveform diagram of FIG. 2 depicts how electrical current supplied by the writing current circuit 42 to the laser diode 34 varies during recording of a single bit of digital data onto the CD or DVD 16. Depending upon specific recording conditions, in conventional drives 10 the electrical current which the writing current circuit 42 supplies to the laser diode 34 when recording onto a DVD at 16X increases from a nominal value of approximately fifty (50) milliamperes (“ma”) at time t0 to as much as several hundred ma at time t1, a time interval of approximately one nanosecond. The maximum electrical current supplied to the laser diode 34, Ip, may be as great as 600 ma. An electrical current supplied to the laser diode 34 which increases too swiftly or overshoots excessively can destroy the CD or DVD 16.
In general, voltage present across an operating laser diode 34 varies depending upon the power of light emitted by the laser diode. For laser diodes 34 used for recording CDs and DVDs, typically the voltage across the laser diode 34 is between 1.7 volts (“V”) and 3.7 V.
A significant performance difference required for a writing current circuit 42 adapted for recording digital data onto a CD and a writing current circuit 42 adapted for recording digital data onto a DVD arises from the smaller size spot 36 written on DVDs. The size of the spot 36 recorded onto DVDs is approximately one-seventh ( 1/7) the size of the spot 36 recorded onto CDs. Consequently, for the same rotation speed of the CD or DVD 16, data must be written seven (7) times faster when recording onto a DVD than when recording onto a CD. Correspondingly, for the same rotation speed the interval during which the light beam heats the spot 36 while writing a single bit of digital data onto a DVD is only one-seventh ( 1/7) of the interval for writing digital data onto a CD. Therefore, for media having similar physical properties the beam of light produced by the laser diode 34 must heat a DVD seven (7) times faster than the beam of light used for recording digital data onto a CD.
Typically, that portion of the writing current circuit 42 which supplies electrical current directly to the laser diode is fabricated using complementary metal oxide silicon (“CMOS”) IC technology. As is known to those skilled in the art, the voltage which may be supplied to a CMOS IC depends upon the thickness of a silicon dioxide (SiO2) insulating layer of the IC that is present between a control gate of metal oxide silicon (“MOS”) field effect transistors (“FET's) included in the CMOS IC and a conducting channel of the MOSFET. As is also known to those skilled in the art, thinning the SiO2 insulating layer of a MOSFET together with other appropriate changes in the MOSFET's structure increases the MOSFET's gain and operating speed, but also lowers the maximum voltage which may be supplied to the CMOS IC. If a 0.5 micron (μ) SiO2 insulating layer exists between the MOSFET's control gate and the conducting channel, then the IC's operation may be energized with a 5.0 V electrical potential. Alternatively, if a 0.33 micron (μ) SiO2 insulating layer exists between the MOSFET's control gate and the conducting channel, then the IC's operation may be energized with only a 3.3 V electrical potential.
To improved MOSFET performance by thinning the SiO2 insulating layer while energizing an IC's operation with a voltage such as 5.0 V which exceeds that permitted for the thin SiO2 insulating layer, it has been known to:                1. fabricate MOSFETs in the core of an IC, such as a microprocessor having one million (1,000,000) or more gates, with a thin SiO2 insulating layer that requires the lower supply voltage;        2. fabricate MOSFETs that surround the IC's core with a thicker SiO2 insulating layer thereby providing MOSFETs that are compatible with the higher supply voltage; and        3. include a voltage regulator circuit in the IC for supplying electrical current to IC's core which reduces the higher supply voltage to the lower voltage compatible with the thin SiO2 insulating layer used in the core's MOSFETs.Disclosure        
An object of the present disclosure is to provide a writing current circuit that permits writing digital data more swiftly.
Another object of the present disclosure is to provide a writing current circuit that supplies to the laser diode of an optical recording device an electrical current that changes smoothly.
Another object of the present disclosure is to provide a writing current circuit that supplies to the laser diode of an optical recording device an electrical current controllably.
Briefly, the disclosed writing current circuit supplies a controlled electrical current to a laser diode included in a drive that is adapted for swiftly recording a DVD. The writing current circuit operates responsive both:                a. to write control digital data for controlling operation of the writing current circuit; and        b. to serial digital data which controls application of the electrical current to the laser diode.The write control digital data specifies at least an amount of electrical current which the writing current circuit controllably supplies to the laser diode. The serial digital data specifies digital data to be recorded on the DVD. Both the write control digital data and the serial digital data are received from the control processor included in the drive. The writing current circuit's operation is energized by an electrical potential applied thereto.        
The writing current circuit includes a plurality of separate current sources. Each of the current sources receives a single output signal from a current control register included in the writing current circuit. The output signal received by each of the current sources from the current control register when in a first state activates the current source for supplying a particular quantity of electrical current to the laser diode. The current source supplies the electrical current to the laser diode via a current output line that connects in the laser diode series with the MOSFET output transistor. When the output signal received by each of the current sources is in a second state, the current source is deactivated for supplying through the MOSFET output transistor the particular quantity of electrical current to the laser diode via the current output line. Advantageously, the MOSFET output transistor included in each of the disclosed current sources has a gate insulating layer which is thinner than the gate insulating layer conventionally used for a MOSFET output transistor that is energized by the electrical potential applied to the writing current circuit.
These and other features, objects and advantages will be understood or apparent to those of ordinary skill in the art from the following detailed description of the preferred embodiment as illustrated in the various drawing figures.